1. Field of the Invention
The present invention relates generally to arrangements for reducing acoustic reflections and diffraction in a loudspeaker and, in particular, to an arrangement for minimizing acoustic reflections and diffraction from the exterior surface of a diaphragm of a loudspeaker transducer.
2. Description of the Relevant Art
A common problem in existing loudspeakers is that the acoustic output from a transducer is reflected and diffracted off of the diaphragm of any other transducers in close proximity. The most common situation is the reflection of a tweeter's output off of the midrange or woofer cone (usually directly below or above the tweeter). Complicating the problem is the fact that the midrange or woofer diaphragm is usually conical in shape. This concave cavity causes large phase shifts of the reflected acoustic signals which, when recombined with the primary (on-axis) signal, will result in large amplitude and phase anomalies (the "cavity effect"). The general perception of these anomalies is manifested as large amounts of timbre-related distortion or coloration.
In the past, manufacturers have dealt with this problem by either of two ways: (1) the use of an acoustically absorbing barrier (12 to 25 mm in height) between the transducers; or (2) the use of a flat diaphragm for the midrange or woofer transducers.
The acoustic barrier approach is partially effective, at least at higher frequencies. However, it does not prevent the problem entirely because it can only absorb a portion of the off-axis output from a transducer. This approach may also have a somewhat unpredictable effect upon the loudspeaker's combined off-axis or polar frequency response. Often the acoustic barrier approach is also unacceptable for aesthetic reasons as a result of its protruding height.
On the other hand, the use of a flat diaphragm for the midrange or woofer transducer will eliminate the "cavity effect" mentioned above but does not eliminate reflection and diffraction. The reflections which remain are far less detrimental, but they still exist. In fact, a more severe problem can sometimes be created by using a flat diaphragm--loss of rigidity. Cone-shaped diaphragms are far superior to flat diaphragms in terms of rigidity and freedom from "breakup" or resonant mode excitation.